State
College, PA (5/1/98)- New genetic detective work putting mammals in
the Cretaceous era may challenge long-held theories of the fall of the
dinosaurs and the rise of warm-blooded animals.
In the largest evolutionary study of gene sequences ever performed,
researchers mined Genbank, the ever-expanding database of gene
sequences maintained by the National Institutes of Health. They scanned
through thousands of vertebrate gene sequences from hundreds of species
looking nuclear genes that develop mutations at a constant rate over time.
By analyzing 658 of these genes they were able to come up with a molecular
timescale for vertebrate evolution.
The new data suggest that modern orders of mammals first evolved when
the continents were separating during the
Cretaceous era about 100 million years ago. This would be much earlier
than some previous estimates based on fossil studies, which link the evolutionary
event to mass extinctions 65 million years ago.
"The evolution of mammals appears to have occurred gradually by the
isolation of breeding groups when the continents broke apart, not suddenly
by the rapid filling of ecological niches left vacant when the dinosaurs
became extinct," reported Blair Hedges, associate professor of biology,
Penn State University. .
"This is the first time we ever have been able to estimate when all
these lifeforms appeared on Earth. Fossils can't
give us this information, partly because there are huge periods of
Earth's history from which not enough fossils have been found to make reliable
estimates," he said.
The traditional method of using fossils to estimate when two species
diverged from their last common ancestor necessarily results in an underestimation,
says Hedges: "The body structure of a fossilized animal had to have
evolved at some earlier date before its lifetime--in many cases, it was
many generations and many millions of years earlier. But genes are different--their
clock-like mutations start ticking away as soon as a new species evolves,
so the molecular clock takes you back to the actual time of origin."
By comparing individual genes in pairs of species, the researchers identified
658 genes from 207 vertebrate species that had accumulated mutations at
a fairly constant rate relative to one another during their evolution.
The scientists then calibrated this molecular clock to an evolutionary
event well established by fossil studies--the divergence of birds and mammals
about 310 million years ago.
"A clock isn't any good unless it is calibrated to a time that everyone
else agrees on," Hedges explains, "and just about everyone agrees on the
date when reptilian ancestors of birds and mammals appeared because it
is based on well-accepted studies of fossils." Using this date as a secure
calibration point--and the mutation rate for each of the constant-rate
genes as a timing device--the researchers were able to determine how long
ago each vertebrate order originated.
While some of the genetic findings supported origin dates similar to
those based on fossil dating, in other cases the genetic clues led back
to a much earlier time. For example, while the fossil record for rodents
sugggests that
mouse and rat split only 10 million years ago, genetic data in this
study indicates that mouse and rat split from their last common ancestor
41 million years ago. At least fivelineages of placental mammals arose
more than 100 million years ago, and most of the modern orders seem to
have diversified before the Cretaceous/Tertiary extinction of the dinosaurs,
the new data suggests.
"We are saying mammals definitely were living on Earth during the Cretaceous
period from 70 to 100 million years ago. We don't yet know what they look
like, but from the genes of their descendants we now know that they were
there."
Paleontologists rarely look for mammalian fossils in rocks formed during
the Cretaceous period. With luck paleontologists may reconsider, and start
looking in these previously neglected strata for mammal fossils during
the time of the dinosaurs, said Hedges.
The research is published in the April 30, 1998, issue of the journal
Nature.
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